1. Field of the Invention
This invention relates to optical sensors and methods of measuring in which the magnitude of a physical quantity is measured by the amount that it alters the polarization of light passed through a sensor; and in particular to elasto-optic sensors which can be used to measure stress induced by pressure or acceleration, and magneto-optic sensors which can be used to measure electric currents. More specifically, the invention is related to such sensors and methods which utilize two wavelengths of light, one of which is polarized and the other of which is unpolarized, with the optical signals multiplexed over a single pair of fiber optic cables and received by a single photodetector.
2. Prior Art
Optical sensors in which a physical quantity alters the polarization of light passed through the sensor element and converted to an electrical signal by a photodetector are well known. In the case of magneto-optic sensors, the electromagnetic field generated by an electric current rotates the plane of polarization of lineraly polarized light passed through magneto-optic material. In elasto-optic sensors, stress induced in elasto-optic material by pressure or acceleration alters the polarization of circularly polarized light. To utilize this latter effect in the measurement of acceleration and pressure, the optical transmission changes in the sensor due to the photoelastic effect have to be separated from the variations due to other effects that cause changes in the photodetector current. Among the effects to be eliminated are variations in the light source efficiency, photodetector sensitivity and dark current as well as both long-term and vibration-induced variations in optical cable and connector losses. Similar signal separation is required for magneto-optic current transformers.
For both accelerometers and current transformers that are restricted to alternating current with bandpaths above frequencies associated with the undesirable variations, frequency discrimination can be used to accomplish the required separation as suggested in U.S. Pat. No. 4,540,937. For magneto-optic direct current transformers as well as for pressure sensors, the above simple method of signal separation can not be utilized. One separation method for the direct current transformers utilizing four optical fiber cables in conjuction with difference/sum calculations from light transmission by two orthogonal polarized light analyzers has been described in our commonly owned U.S. patent application Ser. No. 866,016 entitled "A Direct Current Magneto-Optic Current Transformer".
A second method of signal separation for magneto-optic direct current transformers utilizing two distinct wavelength light sources multiplexed over the same two fiber optic cables and incident on the same photodetector has been described in our commonly owned U.S. Pat. application Ser. No. 717,989 filed on Mar. 29, 1985. The difference in the Verdet coefficients of the magneto optic material at the two wavelengths permits calculations of the average magnetic field intensity and therefore current, as well as its variations, at frequencies below the multiplexing frequency. This approach can also be used for signal separation in elasto-optic pressure gages and accelerometers where the frequency of optical cable and connector vibrations within the desired passband present problems. While such multiple light wavelength elasto-optic accelerometer and pressure gage designs are believed to be satisfactory, a reduction in the complexity of the optical to electrical interface is desirable.